[kwikbreaks]

Try turning off the router and either do a quiet line test (dial 17070 and option 2 iirc) or simply dial a 0. You should get something pretty close to silence. If you try that again with the router powered up and hear noise then it's either a duff filter (rule out by swapping it for another one) or a type of line fault known as a high resistance disconnect - a bad joint which is acting as a rectifier and converting the ADSL RF signal to one you can hear on the line.

I had one of those. 5 Openreach callouts (I just called it in as a voice fault) and no fix despite them claiming to have replaced every line segment back to the exchange. The last bloke spotted it as an ADSL issue and said pursue it through the ISP. They were useless - applied a huge target noise margin slowing the BB down to a crawl (and having zero effect on the noise) then sending out a replacement router to my old address. I lost the will to let them continue guessing and moved to cable instead.